In recent years, so-called jet-propulsion personal watercraft (PWC) have been widely used in leisure, sport, rescue activities, and the like. The personal watercraft is generally configured to suck water under a body from a water intake provided on a hull bottom surface and to pressurize and accelerate the water by an impeller of a jet pump to eject the water rearward of the body. As the resulting reaction, the personal watercraft is propelled forward.
As used in the specification and claims, directions are referenced from the perspective of a rider riding in the personal watercraft, and therefore, travel direction of the personal watercraft is “forward.”
FIG. 6A is a cross-sectional view of an outer peripheral portion of fairing vanes of a conventional pump casing. FIG. 6B is a view showing a detailed structure of an attack angle at a front end portion of the fairing vane shown in FIG. 6A. In these Figures, the left side is forward. As shown in FIG. 6A, an impeller placement part 63 in which an impeller is placed is provided at a front portion of a tubular pump casing 60, and a fairing vane part 64 in which a plurality of fairing vanes 62 are arranged to guide a rotational jet flow pressurized, accelerated and ejected by the impeller and to convert a rotational component thereof into a propulsion force, is provided behind the impeller placement part 63. Each fairing vane 62 provided in the fairing vane part 64 has a structure in which a front end portion thereof is directed toward an entry angle direction of a rotational jet flow generated by the impeller to form an attack angle α and an axis of a rear portion thereof conforms to an axis of the pump casing 60. As shown in FIG. 6B, the attack angle α at the front end of the fairing vane is an angle with respect to the axis of the pump casing 60.
In recent years, large-sized personal watercraft have been developed. Engines mounted in relatively large-sized personal watercraft are designed to output much higher power than the conventional engines. As one example of such high-power engines, an engine equipped with a supercharger which is a supercharging machine for pressurizing air taken into the engine has been manufactured. For the purpose of increasing pump efficiency, the jet pump of the personal watercraft equipped with the high-power engine is expected to reduce pressure loss by minimizing a distance between the impeller disposed in the front portion of the pump casing and the fairing vanes arranged in the rear portion of the pump casing. In addition, the jet pump is expected to increase efficiency by increasing the fairing vanes in number.
However, as the distance between the impeller and the fairing vanes decreases, cavitation is more likely to be generated particularly at a region near outer peripheral portions of the fairing vanes, causing decreasing pump efficiency. In many cases, such cavitation is generated by the fact that the impeller is driven to rotate at a high speed by the high-power engine so as to increase a speed of the water jet flow.
To avoid generation of the cavitation, increasing the attack angle α of the fairing vanes 62 shown in FIG. 6B is effective. However, in the case of the jet pump of the personal watercraft, it is difficult to increase the attack angle α of the fairing vanes 62, considering the fact that the pump casing is mounted in a narrow space and has a small diameter, and the pump casing 60 and the fairing vanes 62 are integrally cast. To be specific, in a case where the fairing vanes 62 are integrally formed inside the pump casing 60 by casting, it is necessary to design the shape of the fairing vanes 62 so that they can be taken out from a casting mold. So, increasing the attack angle of the fairing vanes will make it difficult to take out the fairing vanes from the casting mold.
Furthermore, it is difficult to increase the fairing vanes 62 in number, because a flow passage formed in the pump casing 60 having a small diameter as described above is narrowed, reducing pump efficiency.
As a prior art fairing vane of this type, a fairing vane of a turbine which is cut in straight-line shape from an intermediate point of its back-surface side exit portion to its tip end to inhibit separation of a jet flow is disclosed (see Japanese Laid-Open Patent Application Publication No. Hei. 9-125904). However, this prior art is intended to inhibit separation of the jet flow at the exit portion of the fairing vane, and is incapable of suppressing generation of the cavitation at the front portion of the fairing vane in the jet pump of the personal watercraft.